Light-emitting devices comprising self-emissive thin film light emitting elements such as organic light emitting diodes (OLEDs) represent an attractive technology for flat panel display and solid-state lighting. OLED devices generally can have two formats known as small molecule devices such as disclosed in U.S. Pat. No. 4,476,292 and polymer OLED devices such as disclosed in U.S. Pat. No. 5,247,190. Either type of OLED device may include, in sequence, an anode, an organic EL element, and a cathode. The organic EL element disposed between the anode and the cathode commonly includes an organic hole-transporting layer (HTL), an emissive layer (EL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the EL layer. Tang et al. (Appl. Phys. Lett., 51, 913 (1987), Journal of Applied Physics, 65, 3610 (1989), and U.S. Pat. No. 4,769,292) demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed and device performance has been improved.
Light is generated in an OLED device when electrons and holes that are injected from the cathode and anode, respectively, flow through the electron transport layer and the hole transport layer and recombine in the emissive layer. Many factors determine the efficiency of this light generating process. For example, the selection of anode and cathode materials can determine how efficiently the electrons and holes are injected into the device; the selection of ETL and HTL can determine how efficiently the electrons and holes are transported in the device, and the selection of EL can determine how efficiently the electrons and holes be recombined and result in the emission of light, etc. It has been found, however, that one of the key factors that limits the efficiency of OLED devices is the inefficiency in extracting the photons generated by the electron-hole recombination out of the OLED devices. Due to the high optical indices of the organic materials used, most of the photons generated by the recombination process are actually trapped in the devices due to total internal reflection. These trapped photons never leave the OLED devices and make no contribution to the light output from these devices.
A typical OLED device uses a glass substrate, a transparent conducting anode such as indium-tin-oxide (ITO), a stack of organic layers, and a reflective cathode layer. Light generated from the device is emitted through the glass substrate. This is commonly referred to as a bottom-emitting device. Alternatively, a device can include a substrate, a reflective anode, a stack of organic layers, a top transparent cathode layer, and a transparent encapsulating cover. Light generated from the device is emitted through the top transparent electrode and encapsulating cover. This is commonly referred to as a top-emitting device. In these typical devices, the index of the ITO layer, the organic layers, and the glass is about 2.0, 1.7, and 1.5 respectively. It has been estimated that nearly 60% of the generated light is trapped by internal reflection in the ITO/organic EL element, 20% is trapped in the glass substrate, and only about 20% of the generated light is actually emitted from the device and performs useful functions. Thus, it can be seen that a major shortcoming of OLEDs is that only a small fraction of light generated in the organic layers is emitted from the device. A significant amount of light is trapped by total internal reflection (TIR) because of the relatively large differences in refractive index at the anode-substrate and substrate-air interfaces.
Methods for improving the extraction or out-coupling of light from OLEDs are known in the art. A number of approaches have focused on the substrate-air interface. For example, Moller and Forrest (Journal of Applied Physics, volume 91, page 3324, March 2002 incorporated here as reference) have demonstrated an increase in light output by a factor or 1.5 by attaching a micro-lens array to the glass substrate of an OLED. But there are significant problems with this approach. Preparation of the micro-lens array is by a complex multi-step process involving chemical vapor deposition, photolithography and chemical etching. Also, the micro-lenses do not have the optimum shape. Calculations by Peng et al. (Journal of Display Technology, volume 1, page 278, December 2005 incorporated here as reference) indicate that micro-lenses should have perfectly hemispherical shape for maximum light extraction. Peng et al. describe a process for fabrication of a micro-lens array based on coating a thin layer of photo-resist material on a glass substrate followed by patterning the photo-resist by conventional lithography and then modifying the shape of the photo-resist disks by melting and re-flow. Although the process is an improvement over the method of Moller and Forrest that is reflected in higher light extraction, a factor of 1.85 versus 1.5; there are still problems to be overcome. Melting and re-flow of the photo-resist is difficult to control resulting in significant deviations from hemispherical shape. Furthermore, the distance between lenses is fixed at 1 μm because a smaller spacing or higher resolution is difficult to obtain using this process that is characteristic of “top-down” or conventional micro-fabrication technology. The minimum spacing of 1 μm limits the area fill factor to less than 0.80 for a hexagonally close packed array of hemispherical micro-lenses having diameters of less than 20 micrometers, where the fill factor is defined as the ratio of the area occupied by the micro-lenses to the total area of the surface. Clearly, it is desirable to have a micro-lens array with fill factor close to unity to achieve maximum light extraction.
US2004/0189185 also teaches an OLED device with a micro-lens array. However, once again the micro-lens array is fabricated by conventional micro-fabrication methods such as wet etching and photo-resist re-flow that have the same disadvantages as noted above.
Sun and Forrest (Journal of Applied Physics, volume 91, pages 073106-1 to 073106-6, published online Oct. 11, 2006) describes an OLED device with microlenses fabricated by imprint lithography, wherein a negative microlens array pattern is etched into a glass mold, which is used to imprint a microlens array in a polymethylmethacrylate layer spun-coated on an OLED glass substrate. The described process enables a microlens array to be formed with PMMA, which has a refractive index closely matched to the glass substrate, and a desirably high elastic modulus providing improved scratch resistance compared to previously described microlens arrays formed from PDMS. Formation of a close packed hexagonal array consisting of 6.6 micrometer diameter by 2.2 micrometer high microlenses is reported.
Yabu and Shimomura (Langmuir, volume 21, page 1709, 2005) describe an alternative approach for preparing micro-lens arrays. In this process, a solution of polymer in a volatile organic solvent is cast under humid conditions. Evaporation of the organic solvent under the same humid conditions followed by subsequent evaporation of condensed water droplets from the cast composition results in a polymer film containing a uniform closely packed three dimensional network of spherical pores. A close-packed array of pillar structures is then generated by peeling off the top layer of the film with spherical pores. A polymeric material is subsequently coated over the pillar structure, cured and then released to form a micro-lens array. Yabu and Shimomura do not quantify the fill factor or the shapes of the individual micro-lenses in the array. Furthermore, they do not discuss the effectiveness of the micro-array for light extraction from OLEDs. Also, fabrication of the array involves a large number of steps that may not be suitable for low-cost high volume manufacturing. A simple process requiring less number of steps leading to micro-lens arrays having a high fill factor and hemispherical shaped micro-lenses and the integration of such arrays with OLEDs is still needed.
An additional problem with the micro-lens array of Yabu and Shimomura and other micro-lens arrays in the prior art is that the micro-lens array comprises a precisely ordered array of lenses. An ordered array of lenses in an OLED display can cause significant diffractive artifacts from intense ambient point sources, such as sunlight, or incandescent lamps. It would be desirable to have OLED devices with integrated micro-lens arrays wherein the lenses in the micro-lens arrays are close-packed but randomly distributed.
Srinivasarao et al. (Science, volume 292, page 79, 2001) also describe a process for creating a micro-voided polymer film that involves casting a solution of polymer in a volatile organic solvent in the presence of moist air. Srinivasarao et al. indicate that the shape of the micro-voids in the polymer film depends on the density of the volatile organic solvent relative to water. A film with a three dimensional network of spherical pores as obtained by Yabu and Shimomura is formed if the solvent is less dense than water whereas a film containing only surface cavities is obtained if the solvent has a higher density than water. Srinivasarao mention polymers such as polystyrene containing an end-terminated carboxylic acid group, cellulose acetate and polymethylmethacrylate as being suitable polymers for forming the micro-voided polymer films but do not mention any specific properties to guide the selection of polymers. Furthermore, Srinivasarao et al. do not teach how the micro-voided polymer films are to be used for preparing micro-lens arrays for improved light extraction in OLEDs.
Copending U.S. Ser. No. 11/741,472, the disclosure of which is incorporated by reference herein, describes a simple method for preparing microlens array films with hemispherical shaped microlenses and high fill factor. The process involves forming a solution of an organic solvent polymer in a volatile water-immiscible organic solvent having specific gravity greater than that of water, casting the solution in a humid environment and condensing water droplets on the cast solution, evaporating off the solvent and condensed water droplets to create a first structured polymer film, coating a second fluid polymer composition over the first structured polymer film, curing the second polymer fluid composition while it is in contact with the first structured film to render it solid and create a second structured film comprising a first flat side and a second side with an array of microlenses corresponding to the cavities in the first structured film, separating the second structured film from the first structured film and attaching the flat side of the second structured film to a transparent substrate of a light-emitting device. While the disclosed process produces a microlens array with high fill factor and a random distribution of microlenses having nearly hemispherical shape, the material employed for formation of the second structured film in the disclosed examples has a high hydrophobicity and low surface energy in order to facilitate separation of the cured second structured film from contact with the first structured film. Elastomeric materials such as silicones or natural rubber that are hydrophobic and have low surface energy, however, may not have desirable optical properties in terms of refractive index or light transmission or physical properties in terms of scratch resistance. While it is possible to chemically treat and modify the surface of the first structured film (after it has been prepared and before the second fluid composition is applied on it) to allow materials that are hydrophilic and high surface energy to be released from it, this involves several additional process steps that are not desirable.
An object of this invention is to provide a light-emitting device, such as an OLED device comprising a transparent substrate or cover through which light is emitted, with a micro-lens array having refractive index closely matched to that of the transparent substrate or cover, wherein the micro-lens array has a high fill factor of relatively small microlenses that are randomly distributed, wherein the microlens array is obtainable by a simple low-cost method and the light emitting device demonstrates high light output.